Hureaulite conversion coating as a base for the bonding of rubber to metal

ABSTRACT

A metal-mineral system and method for making and using it. The metal-mineral system creates a bond between a metallic substrate and a top layer, preferably of rubber (to be adhered to). The metal-mineral system includes a fine crystalline coating of hureaulite (manganous iron phosphate) that is formed at less than 80° C. as a conversion coating onto the metallic substrate. The conversion coating passivates the metallic substrate and provides an enhanced surface area of contact that anchors the adhesive to the conversion coating. An adhesive is bonded to the conversion coating. A rubber top layer is applied to the adhesive. The metal-mineral system is resistant to breakage under stress and chemical attack. The method for making the metal-mineral system creates a reduced quantity of sludge with a beneficial effect on waste treatment procedures.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to compositions and processes fordepositing a manganous iron phosphate (“hureaulite”) conversion coatingon a metallic surface. More specifically, the invention relates to suchcompositions and processes that produce, at a temperature not more thanabout 80° C., a conversion coating that is applied to a metallicsubstrate that is suitable for supporting an adhesive to which anorganic coating is affixed.

[0003] 2. Background Art

[0004] Conversion coatings form a base for the adhesion of organiccoating to metallic substrates. As used herein, the term “conversioncoating” refers to the conversion of a metallic surface to anon-metallic, mineral surface. The conversion coating is chemicallyanchored to a metallic substrate, and provides a corrosion-resistantbase.

[0005] The rubber bonding industry utilizes phosphate-based conversioncoatings to insure a strong bond between the metal substrate and anadhesive to which a variety of organic compounds, including rubber andpaint, may be bonded. However, the standard conversion coatings used inthe rubber bonding industry are heavy zinc phosphates orcalcium-modified zinc phosphate conversion coatings which impart largecrystals of hopeite and phosphophyllite to the steel substrate surface.Both of these types of phosphates require elevated temperatures of 92°C. to 98° C. for operation, and are known to generate large amounts ofsludge when used on the typical hardened steel substrates that are usedcommercially. More specifically, heavy zinc phosphating baths form aheavy crystalline conversion coating which consists of a mixture ofhopeite [(Zn₃(PO₄)₂).4H₂O] and phosphophyllite [(Zn₂Fe(PO₄)₂.4H₂O]. Suchphosphating solutions typically require long processing times, hightemperatures, and large amounts of zinc. The crystals deposited arelarge and overlapping.

[0006] Illustrative of the prior art are commonly owned U.S. Pat. Nos.5,595,611; and 5,728,235; and M. S. Boulos & M. Petschel, “Coatings forRubber Bonding and Paint Adhesion,” JOURNAL OF MATLS. ENG. & PERF., Vol.6(4) (August 1997). Each of these is incorporated herein by reference.

SUMMARY OF THE INVENTION

[0007] The invention is a metal-mineral system that creates a bondbetween a metallic substrate and a top layer, preferably an organiccoating, to be adhered thereto. The metal-mineral system includes aconversion coating of manganous iron phosphate (hureaulite) that isformed at about 50° C. to 60° C. The conversion coating is depositedonto the metallic substrate, which it serves to passivate. An adhesiveis affixed to the conversion coating. In use, the conversion coatingprovides an enhanced surface area of contact for affixing the adhesivethereto. Superior performance is achieved by the metal-mineral systemhaving high resistance to breakage and stress and the characteristic ofbeing chemically resistant.

[0008] Thus, the invention describes a conversion coating which impartsa fine crystalline coating of hureaulite and iron-hureaulite to ametallic, preferably, steel substrate. This mineral system results inimproved stress performance as compared to currently used processes.

[0009] Since hureaulite crystals are harder than hopeite-phosphophyllitecrystals, the hureaulite conversion coating forms a base which is lesssusceptible to breakage under stress, and hence improves performance ofthe rubber-to-metal bond. Hureaulite crystals are also more chemicallyresistant than hopeite-phosphophyllite crystals, which also enhancesperformance under a variety of water-based and solvent-based adhesives.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a schematic, cross-sectional view (not to scale) of ametal-mineral system of the present invention; and

[0011]FIG. 2 is a flow chart of the main and optional steps followed inpracticing the method of the disclosed invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0012] Turning first to FIG. 1 of the drawings, there is depicted ametal-mineral system 10 for creating a bond between a metallic substrate12 and a top layer 14 to be adhered to. A fine (about 5-25 microns)crystalline conversion coating 16, preferably of hureaulite or of ironhureaulite is applied to the substrate 12. The manganous iron phosphateconversion coating of the present invention has a formula(Mn,Fe)₅H₂(PO₄)₄.4H₂O (a manganous iron hureaulite). In use, theconversion coating 16 passivates the metallic substrate 12 and providesan enhanced surface area of contact for affixing an adhesive 18 to theconversion coating. The top layer 14 is applied to the adhesive 18. Theresulting metal-mineral system 10 has the characteristics of resistanceto breakage under stress and chemical attack.

[0013] Representative process steps and optional process steps aredepicted in FIG. 2. As shown, the metal substrate is first cleaned in analkaline solution and then rinsed in warm water. Optionally, dependingon the surface type and condition of the substrate, an acid picklingstep may be used, followed by a cold water rinse and a subsequentalkaline cleaning step. Following rinsing in warm water, the metalsurface becomes clean and is oil-free. Next, the oil-free, clean metalsurface is immersed in a nucleating solution of manganese-iron, and amanganese-iron phosphating solution from which the hureaulite crystalsare formed on the metal substrate. This is then followed by cold waterrinse step. Optionally, a corrosion-protective seal can be applied. Thisis followed (optionally) by exposure in a dry-off oven before theadhesive is applied. Following another dry-off exposure step in an oven,the adhesive-coated part may be stored. The part is subsequently bondedto rubber by application of pressure and heat, and then allowed to coolbefore use.

[0014] The hureaulite conversion coating forms at relatively lowtemperatures of about 60° C., which represents a substantial savings inenergy when compared to currently used chemistries that operate at about92° C. to 98° C. An added benefit is the relatively lower amount ofsludge generated, which represents savings in waste-treatment.

[0015] The concentrate chemistry includes that found in the processingbath (Henkel Surface Technologies' commercial product Parco Lubrite (R)LT-10) and its replenishment (Parco Lubrite (R) LT-10 Replenisher).These products have compositions in accordance with the disclosures ofU.S. Pat. Nos. 5,595,611 and 5,728,235 which were referenced earlierherein.

[0016] The principal advantages of the invention are:

[0017] Improved stress performance of the metal-to-rubber bond;

[0018] Lower operating temperature;

[0019] Lower sludge generation;

[0020] Internally accelerated; and

[0021] Provides corrosion protection to the treated work piece.

[0022] Preferably, the processing solution operates at a Q-Value=3.5,with a free acid=2 to 6; and a total acid=35 to 60. As used herein, theterm “Q-value” is defined as the ratio of nitrate concentration tophosphate concentration, or [NO₃]/[PO_(4].)

[0023] The total acid content, consistent with general practice in theart, is measured in “points”, which are defined for the purposes of thisdescription to be equal to the milliliters (“ml”) of 0.1 N NaOH requiredto titrate a 10 ml aliquot sample of the composition to a pH of 8.2(e.g., with a phenolphthalein indicator). The content of “free acid” ofcompositions according to the invention is defined in the same way aspoints of total acid, except at the titration is to a pH of 3.8 (e.g.,with bromophenol blue indicator). Also, as used herein, the term“rubber” refers to any number of natural or synthetic high polymers thatare characterized by elastic recovery after vulcanization with sulfur oranother cross-linking agent.

[0024] It is reported that phosphate baths based primarily on manganeseas coating-forming cations usually form a crystalline species on ferroussurfaces comprised of (Mn,Fe)₅H₂(PO₄)₄.4H₂O. This is a Mn, Fe hureaulitemixed crystal with variable amounts of the interchangeable Mn and Fe. W.Rausch, THE PHOSPHATING OF METALS, ASM, p. 103, which is incorporatedherein by reference.

[0025] Although the adhesion mechanism between the conversion coatingand the substrate is not known with precision, and without being boundby any specific theory, it is reported that the adhesion of manganousiron phosphate coating to metallic substrates stems from a mechanical“keying” of the phosphate crystals into the roughness of a metalsubstrate. Additionally, the structure and epitaxial orientation betweenthe crystals of the phosphate and the grain structure of the base metalalso play a significant role in the adhesion mechanism. W. Raush, supra.

[0026] In the examples below, the rubber bondings studies were conductedon post vulcanized rubber systems where curing adhesives were used tocreate the rubber to metal bond. If desired, water-based adhesives maybe used because of the favorable environmental impact when compared withsolvent-based types of adhesive. Solvent-based adhesives may performbetter than water-based adhesives because the former had the capabilityof solubilizing residual soils on the metal surface. In contrast,water-based adhesives tend not to contribute to surface cleaning.

EXAMPLES

[0027] Examples of rubber bonding adhesion testing using the disclosedhureaulite coating are given in the Tables 1 and 2. A number of steelcoupons were exposed to different surface treatment conversion coatingsand then rubber bonded.

[0028] Table 1 gives the rubber bonding “failure interface” resultsobtained for these coupons when tested by the ASTM-D429 “B” method usinga solvent-based single coat adhesive, 252X. The disclosed hureaulitecoating (“Mn-Phosphate”) was compared with two conventional conversioncoatings normally used in the rubber bonding industry, namely:calcium-modified zinc phosphate (“Zn—Ca Phosphate”) and heavy zincphosphate (“Heavy Zn-Phosphate”). The results of Table 1 show a superiorperformance by the disclosed hureaulite coating. The hureauliteconversion coating could withstand up to 88.2 lb. in⁻² with no failureat the coating-to-metal bond (C/M) or the coating-to-adhesive bond (A/C)interfaces, whereas the other two conversion coatings failed at eitherthe C/M or A/C bond, and at a lower applied force. Preferably, firstfailure occurs at the adhesive-to-rubber (A/R) bond or within the rubberitself (R). TABLE 1 Rubber Bonding Performance of Various ConversionCoatings on Steel Coupons: Failure Using Solvent-Based Single CoatAdhesive 252X¹ Applied Conversion Coating Force Percent Failures By Type(Crystalline) (lb. in⁻²) C/M A/C A/R R Mn-Phosphate/Hureaulite 88.2 100 Mn-Phosphate/Hureaulite 65.1 100  Zn—Ca Phosphate 67.4 15 85 Zn—CaPhosphate 60.7 25 75 Heavy Zn-Phosphate 72 20 80 Heavy Zn-Phosphate 7120 80 Grit Blasted — Grit Blasted — Untreated 51.7 90 10 Untreated 51.975 25

[0029] Table 2 gives the rubber bonding “failure interface” resultsobtained for these coupons when tested by the ASTM-D429 “B” method usinga solvent-based double coat adhesive, 205/252X. The disclosed hureaulitecoating “(Mn-Phosphate)” was again compared with two conventionalconversion coatings normally used in the rubber bonding industry:calcium-modified zinc phosphate “(Zn—Ca Phosphate)” and heavy zincphosphate (“Heavy Zn-Phosphate)”. The results of Table 2 show favorableperformance by the disclosed hureaulite coating when compared with thetwo conversion coatings. TABLE 2 Rubber Bonding Performance of VariousConversion Coatings on Steel Coupons: Failure Interface UsingSolvent-Based Double Coat Adhesive 205/252X Applied Force PercentFailures By Type Conversion Coating (lb. in⁻²) C/M A/C A/R RMn-Phosphate/Hureaulite 92.9 100 Mn-Phosphate/Hureaulite 89.9 100 Zn—CaPhosphate 85.1 100 Zn—Ca Phosphate 70.0 100 Heavy Zn-Phosphate 95 100Heavy Zn-Phosphate 113 100 Grit Blasted 96 100 Grit Blasted 100 100Untreated 71.5 100 Untreated Substrate 64.9 100

[0030] These examples demonstrate superior adhesion by the disclosedpolycrystalline conversion coating, which imparts a fine crystallinemorphology to the metallic surface and markedly improves the adhesion ofthe top layer.

[0031] While embodiments of the invention have been illustrated anddescribed, it is not intended that these embodiments illustrate anddescribe all possible forms of the invention. Rather, the words used inthe specification are words of description rather than limitation, andit is understood that various changes may be made without departing fromthe spirit and scope of the invention.

What is claimed is:
 1. A metal-mineral system for creating a bondbetween a metallic substrate and an adherent top layer, themetal-mineral system comprising: a fine crystalline conversion coatingof manganous iron phosphate formed at less than 80° C. onto the metallicsubstrate, the conversion coating serving to passivate the metallicsubstrate and provide an enhanced surface area of contact for anchoringan adhesive to the conversion coating; an adhesive bonded to theconversion coating; and a rubber top layer applied to the adhesive, themetal-mineral system being resistant to breakage under stress andchemical attack.
 2. The metal-mineral system of claim 1, wherein themanganous iron phosphate coating comprises: an average crystal size ofabout 5 to about 25 microns.
 3. The metal-mineral system of claim 2,wherein the manganous iron phosphate coating comprises: an averagecrystal size of about 10 to about 20 microns.
 4. The metal-mineralsystem of claim 1 wherein the rubber is selected from the groupconsisting of natural and synthetic high polymers.
 5. The metal-mineralsystem of claim 1, wherein the metallic substrate comprises steel. 6.The metal-mineral system of claim 1, wherein the metallic substratecomprises zinc.
 7. The metal-mineral system of claim 1, wherein themetallic substrate comprises a ferrous metal
 8. The metal-mineral systemof claim 1, wherein the metallic substrate comprises a galvanized steel.9. The metal-mineral system of claim 1, wherein the fine crystallineconversion coating is formed at a temperature between 50° C.-80° C. 10.The metal-mineral system of claim 1, wherein the fine crystallineconversion coating is formed at a temperature between 50° C.-60° C. 11.The metal-mineral system of claim 1, wherein the conversion coating ofmanganous iron phosphate comprises iron hureaulite.
 12. A method ofmaking a metal-mineral system for creating a bond between a metallicsubstrate and an adherent top layer, comprising the steps of: preparingan oil-free, clean metal surface; exposing the oil-free, clean metalsurface to a nucleating solution to form a modified metal surface;immersing the modified metal surface in a manganese-iron phosphatingbath at less than 80° C. to form a fine crystalline conversion coatingof manganese iron phosphate upon the modified metal surface; bonding anadhesive to the conversion coating; and applying a rubber top layer tothe adhesive, the metal-mineral system have the characteristics ofresistance to breakage under stress and chemical attack.
 13. The methodof claim 12 wherein the temperature of the manganese-iron phosphatingbath is at an average temperature between 50° C.-80° C.
 14. The methodof claim 12 wherein the temperature of the manganese-iron phosphatingbath is at an average temperature between 50° C. -60° C.
 15. The methodof claim 12 wherein the step of bonding the adhesive includes bondingthe adhesive relatively uniformly over the entire metallic substrate.16. The method of claim 12, wherein the processing solution operates ata Q-value of about 3.5, with a free acid amount between 2 and 6 points,and a total acid amount between 35 and 60 points.
 17. A method of usingthe metal-mineral system of claim 1 comprising the steps of: providing ametal-mineral system according to the method of claim 12; and deployingthe system in a rubber bonding environment where the system hascharacteristics of resistance to breakage, stress and chemical attack,while generating relatively low amounts of sludge during itspreparation.